Color Modes

The color mode determines how colors combine based on the number of channels in a color model.
RGB

millions of colors (web)
CMYK

millions of colors (print)
Index

256 colors (web)
Grayscale

256 grays (web)
Bitmap

2 colors
More information about color modes is here.

To check or change image color mode in Photoshop go to the Menu bar:
Image > Mode > choose the mode.